C. Galdi

Analysis of GNSS-R Delay-Doppler Maps from the UK-DMC satellite over the ocean

A study of the retrieval of sea surface roughness using Global Navigation Satellite System-Reflectometry (GNSS-R) from satellite is presented. Delay- Doppler Maps (DDMs) from the SSTL UK-DMC satellite are analyzed to retrieve directional Mean Square Slopes (MSSs). Results are compared to theoretically-derived MSSs and in situ measurements from co-located buoys of the National Data Buoy Center (NDBC), showing good agreement in most cases. Here, the whole DDM, a more complete source of information, is exploited for the first time using satellite GNSS-R data. These are potentially able to provide high spatial and temporal sampling, and therefore offer an improved way to observe wind and waves by means of a very modest instrument.

Cooperative sensor networks for voltage quality monitoring in smart grids

The paper intends to give a contribution toward the definition of a fully decentralized voltage quality monitoring architecture by proposing the employment of self organizing sensor networks. According to this para-digm each node can assess both the performances of the monitored site, computed by acquiring local information, and the global performances of the monitored grid section, computed by local exchanges of information with its neighbors nodes. Thanks to this feature each node could automatically detect local voltage quality anomalies. Moreover system operator can assess the system voltage quality index for each grid section by inquiring any node of the corresponding sensors network without the need of a central fusion center acquiring and processing all the node acquisitions. This makes the overall monitoring architecture highly scalable, self-organizing and distributed.

Simulation of L-Band Bistatic Returns From the Ocean Surface: A Facet Approach With Application to Ocean GNSS Reflectometry

We present the implementation of a facet-based simulator to investigate the forward scattering of L-band signals from realistic sea surfaces and its application to spaceborne ocean Global Navigation Satellite System (GNSS) Reflectometry. This approach provides a new flexible tool to assess the influence of the ocean surface roughness on scattered GNSS signals. The motivation stems from the study by Clarizia , which revealed significant differences between delay-Doppler maps (DDMs) obtained from UK-DMC satellite data and DDMs simulated with the Zavorotny-Voronovich (Z-V) model. Here, the scattered power and polarization ratio (PR) are computed for explicit 3-D ocean wave fields, using a novel implementation of the Kirchhoff approximation (KA), which we call the Facet Approach (FA). We find that the FA is consistent with the full KA and the Geometrical Optics (GO) used in the Z-V model, while being less computationally expensive than the KA and able to represent polarization effects not captured by the GO. Instantaneous maps of the bistatic normalized radar cross section computed with the FA show clear patterns associated with the underlying waves. The wave field is particularly visible in the PR, indicating that the scattering is generally dominated by the HH component, particularly from ocean wave troughs. Polarization effects show, for the first time, a strong correlation to the explicit sea surface from which the scattering originated. DDMs of the scattered power computed with the FA reveal patchy patterns and power distributions that differ from those obtained with Z-V and show closer similarities with observed DDMs from UK-DMC.

Synthetic Aperture Radar Processing with GPGPU

This article focuses on methodologies with recurrent use to code examples that try to couple with the flow of the main steps of the SAR processing. The possibility to be comprehensive was prevented by the wide scenario of variations of the focusing algorithm as well as the spread of applications. The reader should look at this work as a sample of possibilities offered by this new technology and a collection of suggestions and considerations that may guide to new applications and horizons.

GNSS-R Altimeter Based on Doppler Multi-looking

Measuring ocean mesoscale variability is one of the main objectives of next generation satellite altimeters. Current radar altimeters make observations only at the nadir sub-satellite ground track, which is not sufficient to sample the ocean surface with the required spatial and temporal sampling. The GNSS-R concept has been proposed as an alternative observation system in order to overcome this limitation, since it allows performing altimetry along several points simultaneously over a very wide swath. Latest proposed GNSS-R altimeter configurations allow measuring sea height with an accuracy of few decimeters over spatial scales of 50-100 km, by means of a single-pass. This paper proposes an innovative processing and retracking concept for GNSS-R altimeters based on the acquisition of the full delay-Doppler map (DDM), which allows to acquire multiple waveforms at different Doppler frequencies, whose footprints are located outside the typical pulse-limited region. The proposed processing adapts the Synthetic Aperture Radar (SAR) delay-Doppler concept of spaceborne radar altimeters for use in a GNSS-R system. This processing yields additional multi-look with respect to conventional GNSS-R concepts and translates into an improvement of the altimetry performance estimated to be at least 25%-30%, and even higher, depending on the wanted along-track spatial resolution. The proposed processing can also provide measurements with high spatial resolution at best possible performance, and more generally, offers various possibilities for optimal trade-off between spatial-resolution and height estimation accuracy.

CFAR detection of extended objects in high resolution SAR images

Presents a processing scheme for the constant false alarm rate (CFAR) detection of extended objects embedded in non-Gaussian disturbance. The proposed receiver exploits some relevant properties of the location-scale distributions for ensuring constant false alarm against Weibull clutter. The system has been specifically conceived for operating on high-resolution SAR images where space processing (but not time processing) is allowed.

Destriping MODIS data using IFOV overlapping

In this paper, we present a new destriping algorithm for MODIS images. The algorithm takes advantage of the observation redundancy due to the bow-tie effect. It estimates the equalization curve from single fields of view observed by multiple detectors. Results show that the strategy is correct in those regions where equalization curve is estimated in the low to moderate range of radiance

Stochastic Modeling and Simulation of Delay–Doppler Maps in GNSS-R Over the Ocean

A stochastic model for delay-Doppler map (DDM) simulation from global navigation satellite system reflectometry (GNSS-R) systems is presented. The aim is to provide a useful tool for investigating the performance of estimation and retrieval algorithms that are based on finite time series. The scattering inside a delay-Doppler cell is modeled as the sum of a random number of contributions from inner specular points that, as the mean number of such contributions gets larger, tends to a compound-Gaussian process. The statistical averages reveal that the model is fully consistent with the previous results provided by Zavorotny and Voronovich. Numerical simulations of large airborne and spaceborne DDMs are easily practicable and show the clear patterns due to signal fluctuations and thermal noise that fade away when the number of averaged observations increases. Comparisons with TechDemoSat-1 data show that the model and the simulation scheme provide accurate realizations of the onboard-processed DDMs.

The Unmixing of Atmospheric Trace Gases From Hyperspectral Satellite Data

A new approach for the retrieval of the vertical column concentrations of trace gases, from hyperspectral satellite reflectances, is presented. The investigation moves from the general rationale of independent component analysis, but the constraint of perfect independence among sources is replaced by a minimum dependence concept that proves more reasonable for the application at hand. The unmixing of the gas spectra and their concentrations is achieved from linear mixtures obtained from the logarithm of the spectral reflectance. After a proper preprocessing stage aimed at reducing major residual dependences caused by atmospheric scattering, trace-gas retrieval is carried out through a minimization of a statistical cost function, subject to the physical constraint that the resulting spectra must be nonnegative. The experimental analysis relies on the retrieval of sulfur dioxide during volcanic emissions using data from the National Aeronautics and Space Administration Ozone Monitoring Instrument. To validate the procedure, reference reflectance spectra having a known profile of sulfur dioxide are generated with the MODerate resolution atmospheric TRANsmission software, and the retrieved concentration is compared with the theoretical one. Performance in the presence of shot and detector noise has also been analyzed starting from pure simulated spectral reflectances.

Simulation of GNSS-R returns for delay-DOPPLER analysis of the ocean surface

We present a new approach to the retrieval of sea surface roughness using GNSS-R. The steps through the simulation of the whole end-to-end microwave scattering of GNSS signals from the sea surface are explained, with emphasis on how to generate a linear sea surface and to implement the Kirchhoff Approximation (KA), as the large-scale part of the full scattering model. We illustrate some examples of radar cross sections calculated using the Kirchhoff scattering model, and how they change with respect to different polarizations. Their variations with geometry, sea state and spatial resolution are investigated and discussed.